Selective production of o-alkylphenols

ABSTRACT

In a method for the production of o-alkyl phenols by conversion of phenol with an alkanol at elevated temperature in the gas phase in the presence of a metal catalyst, the conversion takes place in at least two stages wherein the alkanol/phenol molar ratio in each reaction stage is set to a value of approximately ≦0.4; a clear increase in the selectivity for the o-alkyl phenol is obtained.

[0001] The invention relates to a multistage method for the o-alkylationof phenol by conversion of phenol with an alkanol at elevatedtemperature in the gas phase in the presence of an acidic metal oxidecatalyst.

[0002] Pure o-alkyl phenols are important compounds which are used inlarge quantities as starting material for organic chemical syntheses.Pure o-cresol (2-methyl phenol) is especially used for the production ofpesticides.

[0003] o-cresol can be obtained by methylation of phenol with methanolin the gas or liquid phase. As a result of the low reactivity ofmethanol, the conversion takes place at elevated temperature in thepresence of a catalyst. Metal oxide catalysts such as aluminium oxide,silicon dioxide/aluminium mixed oxide and magnesium oxide are used ascatalysts. The choice of reaction temperature is made as a function ofthe catalyst used in a range of 250 to 460° C. Thus, magnesium oxideexhibits a high selectivity for o-cresol in a temperature range of 420to 460° C. whereas γ-aluminium oxide catalyses the methylation of phenolat a temperature of 200 to 400° C.

[0004] Also known is the production of o-cresol as a side product in thesynthesis of 2,6-dimethyl phenol and the subsequent isolation ofo-cresol by additional purification steps. Methods for the production ofo-cresol with further evidence are described in H. G. Franck, J. W.Stadelhofer, INDUSTRIELLE AROMATENCHEMIE, p. 170-177, SPRINGER VERLAG1987.

[0005] DE 27 56 461 A1 describes a generic method which is carried outat a temperature of 250 to 330° C. using alumina as the catalyst. Ano-cresol yield of up to 26% is obtained with a methanol to phenol ratioof 0.5:1. The product contains an approximately 6% fraction of2,6-dimethyl phenol.

[0006] At high phenol conversions high fractions of 2,6-dimethyl phenolare always obtained in addition to o-cresol. It is difficult to obtaino-cresol selectively by the methylation of phenol. Considerablequantities of m-cresol and p-cresol or higher alkylated products usuallyoccur as side products.

[0007] The high fraction of side products is common to all known methodsfor the industrial synthesis of o-alkyl phenols.

[0008] The object of the invention is thus to provide a method for theproduction of o-alkyl phenol which is as selective as possible and canbe carried out on an industrial scale.

[0009] This object is achieved by o-alkylation of phenol with an alkanolat elevated temperature in the gas phase in the presence of a metaloxide catalyst, in which the conversion is carried out in at least tworeaction stages and the alkanol/phenol molar ratio is maintained atapproximately ≦1 over the entire method.

[0010] The method according to the invention can be carried out, forexample, in two to five stages. The method takes place especiallypreferably in three stages. Each conversion stage can be carried out ina different reactor. However, it is also possible to carry out severalreaction stages in a single reactor. In this procedure severalconfigurations of the active catalyst separated spatially from oneanother are accommodated in the reactor. Between the catalystconfigurations can be arranged zones with catalyst of lower activity orwithout catalyst.

[0011] The work forming the basis of this invention has shown thatanisole is formed during the methylation of phenol and o-cresol isformed, on the one hand, by alkylation of phenol with anisole and, onthe other hand, by intramolecular rearrangement of anisole to o-cresol.This finding is new and contradicts the findings so far. The newlydiscovered reaction sequence is shown in FIG. 1.

[0012] It has surprisingly been found that multistage implementation ofthe method under the afore-mentioned conditions yields a significantlyenhanced selectivity of the reaction for o-cresol and an associatedincrease in the yield of this compound. It is assumed that the highselectivity for o-alkylations is achieved by the low localconcentrations of methanol and therefore also of anisole, required inthis procedure. It is advantageous that the highly exothermic reactioncan be much better controlled by distribution over two, especially threereactors. The formation of so-called hot spots is thereby suppressed.

[0013] As a result of the multistage execution of the reaction, thealkanol to phenol ratio in each reactor or in each reaction stage can beset especially low. The phenol conversion is thereby limited and aparticularly high selectivity for the o-alkyl phenol, for example,o-cresol can be achieved. The molar ratio of alkanol to phenol over theentire method is preferably set to 0.9, especially preferably to 0.6 ora value in between. Thus, the molar ratio of alkanol to phenol in athree-stage process is preferably 0.3 to 0.2 in each reaction stage.

[0014] Expressed alternatively, the phenol conversion can be set to avalue of approximately 35 to 43%, for example, 38 to 42% in order toachieve the desired high selectivity. Suitable catalysts forimplementing the method according to the invention are acid metal oxidesand their mixtures. Such metal oxides are, for example, aluminium oxide,silicon dioxide/aluminium mixed oxide and magnesium oxide. Especiallypreferable is γ aluminium oxide. The surface area of the catalysts ispreferably approximately 250 m²/g and more, especially preferably 250 to300 m²/g. These catalysts are produced by known methods, for example, byammoniacal hydrolysis of aluminium nitrate and subsequent separation,drying and calcination of the precipitate obtained (J. Amer. Chem. Soc.82 (1960) 2471).

[0015] The catalyst can be arranged in the usual form, for example, as afixed bed, fluid bed or fluidized bed. The catalyst is preferablyarranged in a fixed bed.

[0016] Alkanols to be used according to the invention are especiallyC₁₋₄ alkanols, i.e., methanol, ethanol, n-propanol, isopropanol,n-butanol and isobutanol.

[0017] The method according to the invention can be implemented in atemperature range of 250 to 400° C. When γ aluminium oxide is used asthe catalyst, the temperature in the reactor is preferably 300 to 400°C., especially preferably 300 to 340° C., for example, 330° C.

[0018] An embodiment of the method according to the invention usingmethanol as the alkylation agent is described subsequently as anexample. The method is carried out in three stages.

[0019] For this purpose phenol is fed into a mixer/vaporiser via ametering device. Methanol is supplied to the same mixer/vaporiser via ametering device. The mixture of initial compounds is fed into a tubularreactor heated to 330° C. The flow from the reactor contains the initialcompound phenol as well as the products and can be withdrawn at thebottom of the reactor. The flow is fed to another mixer/vaporiser whichis positioned before another tubular reactor. The methanol/phenol ratiorequired for the method according to the invention is adjusted again inthis mixer and the mixture obtained is fed into the second reactor. Theflow from the second reactor is either fed into a cooler or into a thirdmixer/vaporiser which is connected before a third reactor. Themethanol/phenol ratio required for the method according to the inventionis adjusted again in the third reactor and the mixture obtained is fedinto the third reactor.

[0020] The flow from the third reactor, if appropriate, from the secondreactor, is condensed in a cooler and the condensate is fed to a tank.

[0021] The processing of the raw alkylate can preferably take place bycontinuous rectification in a system consisting of three distillationcolumns connected one behind the other.

[0022] In the first column with, for example 20 to 35 practical plateswhich are operated at normal pressure, the reaction water is separatedat the head of the column at a head temperature of 90 to 100° C. Thewater contains small amounts of phenol, alkyl phenols and anisole sincethese compounds distill azeotropically with the water.

[0023] The sump of the first column is fed continuously into the secondcolumn. This column has approximately 100 practical plates and is alsooperated at normal pressure. At the head of the column phenol with smallfractions of o-cresol is withdrawn at a head temperature of 180 to 185°C. This mass flow can be supplied to the alkylation stage again as rawmaterial.

[0024] The phenol-free sump of the column 2 continually enters the feedto the third column with approximately 70 to 95 practical plates at 300mbar head pressure and approximately 145 to 155° C. head temperature.Pure o-cresol having a purity of >99.5% can be drawn off at the head ofthe column.

[0025] The sump of the column which contains small fractions ofo-cresol, can be used as raw material to produce cresol/xylenolmixtures.

[0026] o-alkylated compounds to be obtained according to the inventionare cresol and the ethyl, n-propyl, isopropyl, n-butyl and isobutylderivatives of phenol.

[0027] The following examples serve to explain the invention in greaterdetail.

EXAMPLE 1

[0028] It is known from the literature that anisole forms o-cresol byintramolecular rearrangement. For this reason it was taken as thestarting point that a high concentration of anisole should be built upto enhance the product yield. For this purpose pure anisole was fed intoa tubular reactor at a temperature of 330° C. at an LHSV (liquid hourlyspace velocity) of 1.25 h⁻¹ in a single-stage method. A γ aluminiumoxide with a surface area of approximately 250 m²/g is used as thecatalyst. The products obtained and their concentration in the productmixture are given in the following Table 1. TABLE 1 CompoundConcentration % Anisole 7.1 Phenol 32.2 o-cresol 28.6 2,6-xylenol 16.12,3,6-trimethyl phenol 4.0 Pentamethyl phenol 3.3 Anisole conversion92.9% o-cresol selectivity 30.8%

[0029] The results in the table show that anisole with a 92.9%conversion is highly reactive under the selected conditions. Theconcentration of phenol and higher alkylated phenols such as 2,6-xylenoland 2,3,6-trimethyl phenol shows that only a portion of the anisole isrearranged into o-cresol. Most of the anisole reacts as an alkylationagent.

[0030] In order to confirm this supposition, an alkylation experimentwas carried out under the same conditions as those described previously,in which methanol was completely replaced by anisole. The resultsobtained are given in Table 2. TABLE 2 Alkylation agent Anisole MethanolCompound Concentration % Concentration % Anisole — — Phenol 76.8 72.3o-cresol 18.8 20.5 2,6-xylenol 2.6 3.8 2,3,6-trimethyl — 0.4 phenolo-cresol 81.0% 74.0% selectivity

[0031] The values show that the behaviour of methanol is similar to thatof anisole. The use of anisole even shows a somewhat higher selectivitythan when methanol is used as the alkylation agent.

[0032] It can be postulated that the selectivity for o-cresol isapproximately the same for both alkylation agents. The slightly highero-cresol selectivity of anisole will be attributable to the fact thatthe rearrangement of anisole to form o-cresol takes place at the sametime as the alkylation of phenol by anisole.

[0033] In order to study the fraction of o-cresol which is produced byintramolecular rearrangement, the same experiment was carried out using4-methyl anisole as the model substance. The concentration of theindividual products in the product mixture [%] and the 4-methyl anisoleconversion are given in Table 3. TABLE 3 Compound Concentration % Phenol57.7 o-cresol 12.8 p-cresol 18.6 2,6-xylenol 1.5 2,4-xylenol 6.44-methyl anisole conversion 100%

[0034] The results show that in addition to o-cresol, significantconcentrations of p-cresol and 2,4-xylenol were obtained. p-cresol isformed when 4-methyl anisole acts as an alkylation agent. 2,4-xylenol isthe product of the intramolecular rearrangement of 4-methyl anisole. Thecalculation shows that approximately 70% of the 4-methyl anisole acts asan alkylation agent and approximately 30% is rearranged to form2,4-xylenol. It is assumed that when anisole is used as the alkylationagent, the same ratios exist.

EXAMPLE 2

[0035] Phenol was pumped with methanol into the reactor at a reactortemperature of 330° C. with a methanol/phenol molar ratio of 0.2 and anLHSV of 3.75 h⁻¹. An γ aluminium oxide with a surface area ofapproximately 250 m²/g is used as the catalyst. In the following secondstage methanol was supplied in the molar ratio of 0.2 to the flow fromthe first stage and the alkylation continued. A similar procedure wasadopted to carry out the third alkylation stage.

[0036] For comparison, phenol with methanol was converted in asingle-stage tubular reactor at a reaction temperature of 330° C. with amethanol/phenol molar ratio of 0.6 at an LHSV of 1.25 h⁻¹. The samecatalyst was used. In total the methanol/phenol ratio and the LHSV isthus the same as in the three-stage reaction. The product concentrationin the product mixture, the phenol conversion and the selectivity foro-cresol for both reactions are given in Table 4. TABLE 4 Single-stageThree-stage Compound alkylation alkylation Anisole 0.02 0.9 Phenol 53.758.6 o-cresol 27.9 29.4 m/p-cresol 1.5 0.8 2,6-xylenol 9.5 6.82,4/2,5-xylenol 2.0 0.9 2,3,6-trimethyl 1.7 0.9 phenol Phenol conversion46.3 41.4 o-cresol 60.3 71.1 selectivity

[0037] These results show that in the multistage method the selectivityfor o-cresol is significantly higher than in the single-stage methodalthough the phenol conversion in the three-stage method is similar. Thereason for this is that the quantity of side products in thesingle-stage method is significantly higher than that in the three-stagemethod. Although the anisole content of 0.9% in this experiment is notyet optimal, an increase in the selectivity for o-cresol from 60.3% inthe single-stage method to 71.1% in the three-stage method is obtained.

1. A method for the production of o-alkyl phenols by conversion ofphenol with an alkanol at elevated temperature in the gas phase in. thepresence of a metal catalyst, characterised in that the conversion iscarried out in at least two stages and the alkanol/phenol molar ratio ineach reaction stage is set to a value of approximately ≦0.4.
 2. Themethod according to claim 1, characterised in that the conversion iscarried out in three stages.
 3. The method according to claim 1 or 2,characterised in that the alkanol/phenol molar ratio in each reactionstage is set to a value of approximately 0.2 to 0.4.
 4. The methodaccording to claim 3, characterised in that the alkanol/phenol molarratio in each reaction stage is set to a value of approximately 0.3. 5.The method according to one of claims 1 to 4, characterised in that thephenol conversion during the reaction is set to 35 to 43% in each stage.6. The method according to one of claims 1 to 5, characterised in thatmethanol is used as alkanol.
 7. The method according to one of claims 1to 6, characterised in that an y aluminium oxide having a surface areagreater than 250 m²/g is used as the catalyst.
 8. The method accordingto claim 7, characterised in that the reaction is carried out in atemperature range of 300 to 400° C.
 9. The method according to one ofclaims 1 to 8, characterised in that the product mixture obtained afteralkylation is separated by distillation to obtain the desired product.